Polyolefins produced in a high pressure (HP) process are widely used in demanding polymer applications wherein the polymers must meet high mechanical and/or electrical requirements. For instance in power cable applications, particularly in medium voltage (MV) and especially in high voltage (HV) and extra high voltage (EHV) cable applications the electrical properties of the polymer composition has a significant importance. Furthermore, the electrical properties may differ in different cable applications, as is the case between alternating current (AC) and direct current (DC) cable applications.
A typical power cable comprises a conductor surrounded, at least, by an inner semiconductive layer, an insulation layer and an outer semiconductive layer, in that order.
Space Charge
There is a fundamental difference between AC and DC with respect to electrical field distribution in the cable. The electric field in an AC cable is easily calculated since it depends on one material property only, namely the relative permittivity (the dielectric constant) with a known temperature dependence. The electric field will not influence the dielectric constant. On the other hand, the electric field in a DC cable is much more complex and depends on the conduction, trapping and build-up of electric charges, so called space charges, inside the insulation. Space charges inside the insulation will distort the electric field and may lead to points of very high electric stress, possibly that high that a dielectric failure will follow. Preferably there should be no space charges present as it will make it possible to easily design the cable as the electric field distribution in the insulation will be known.
Normally space charges are located close to the electrodes; charges of the same polarity as the nearby electrode are called homocharges, charges of opposite polarity are called heterocharges. The heterocharges will increase the electric field at this electrode, homocharges will instead reduce the electric field.
Electrical Conductivity
The DC electrical conductivity is an important material property e.g. for insulating materials for HV DC cables. First of all, the strong temperature and electric field dependence of this property will influence the electric field distribution via space charge build-up as described above. The second issue is the fact that heat will be generated inside the insulation by the electric leakage current flowing between the inner and outer semiconductive layers. This leakage current depends on the electric field and the electric conductivity of the insulation. High conductivity of the insulating material can even lead to thermal runaway under high stress/high temperature conditions. The conductivity must therefore be kept as low as possible to avoid thermal runaway.
Compressor Lubricants
HP process is typically operated at high pressures up to 4000 bar. In known HP reactor systems the starting monomer(s) need to be compressed (pressurised) before introduced to the actual high pressure polymerisation reactor. Compressor lubricants are conventionally used in the hyper-compressor(s) for cylinder lubrication to enable the mechanically demanding compression step of starting monomer(s). It is well known that small amounts of the lubricant normally leaks through the seals into the reactor and mixes with the monomer(s). In consequence the reaction mixture contains traces (up to hundreds of ppm) of the compressor lubricant during the actual polymerisation step of the monomer(s). These traces of compressor lubricants can have an effect on the electrical properties of the final polymer.
As examples of commercial compressor lubricants e.g. polyalkylene glycol (PAG): R—[CxRyHz—O]n—H, wherein R can be H or straight chain or branched hydrocarbyl and x, y, x, n are independent integers that can vary in a known manner, and lubricants based on a mineral oil (by-product in the distillation of petroleum) can be mentioned. Compressor lubricants which are based on mineral oils that meet the requirements set for the white mineral oil in European Directive 2002/72/EC, Annex V, for plastics used in food contact, are used e.g. for polymerising polymers especially for the food and pharmaceutical industry. Such mineral oil-based lubricants contain usually lubricity additive(s) and may also contain other type of additive(s), such as antioxidants.
WO2009012041 of Dow discloses that in high pressure polymerisation process, wherein compressors are used for pressurising the reactants, i.e. one or more monomer(s), the compressor lubricant may have an effect on the properties of the polymerised polymer. The document describes the use of a polyol polyether which comprises one or none hydroxyl functionality as a compressor lubricant for preventing premature crosslinking particularly of silane-modified HP polyolefins. WO2009012092 of Dow discloses a composition which comprises a HP (i) polyolefin free of silane functionality and (ii) a hydrophobic polyether polyol of PAG type wherein at least 50% of its molecules comprise no more than a single hydroxyl functionality. The component (ii) appears to originate from a compressor lubricant. The composition is i.a. for W & C applications and is stated to reduce dielectrical losses in MV and HV power cables, see page 2, paragraph 0006. In both applications it is stated that hydrophilic groups (e.g. hydroxyl groups) present in the compressor lubricant can result in increased water uptake by the polymer which in turn can increase electrical losses or, respectively, pre-mature scorch, when the polymer is used as a cable layer material. The problems are solved by a specific PAG type of lubricant with reduced amount of hydroxyl functionalities.
There is a continuous need in the polymer field to find polymers which are suitable for demanding polymer applications such as wire and cable applications with high requirements and stringent regulations.